In recent years, cylindrical dry cells were changed from a paper jacket type to a metal jacket type, and their resistance to leakage and expansion was much improved and their cell performances were significantly enhanced.
An outer metal jacket for such dry cell is formed by joining a rectangular metal sheet member 1, as shown in FIG. 12 of the accompanying drawings, at both side edges 1a and 1b to make a cylindrical shape and folding its lower rim 1c inwardly. The plan view and the bottom view of the thus formed metal jacket 2 are shown in FIGS. 13 (A) and 13 (B), respectively. Into the outer metal jacket 2, various components for constituting an electrochemical cell system are incorporated, and ultimately the upper rim 1d is folded inwardly to make a dry cell, of which the inside is made air-tight.
For joining both side edges 1a and 1b of said plate 1, there are usually adopted a double-locking system wherein said edges are folded in a double layered or overlapped state, a butting system wherein said edges are butted against each other, etc. In addition, there is recently developed a laser-welding system which assures firm joining.
For comparison, a metal jacket 3 of the double-locking system and a metal jacket 4 of the laser-welding system are respectively shown in FIGS. 14 (A) and 14 (B), and a metal jacket 4 of the laser-welding system and a metal jacket 5 of the butting system are respectively shown in FIGS. 15 (A) and 15 (B). As understood from such comparison, the laser-welding system jacket 4 does not have any inward protrusion like the double-locking part 3a in the double-locking system jacket 3 and therefore can accommodate an electrochemical cell system with a zinc can having an outer diameter which is larger by the size (.alpha.) of the double-locking part 3a than a cell with double-locking part. Thus, the laser-welding system jacket 4 makes it possible to provide a dry cell having a larger capacity. Also, the laser-welding system jacket 4 has a higher tensile strength in the radial direction than the butting system jacket 5, and therefore the former does not produce an opening at the butting part as seen in the latter even on excessive discharge.
When, however, a dry cell comprising a laser-welding system jacket firmly joined at the butting part is used improperly, for example, under charge or left at a high temperature such as 85.degree. C. or more, the electrochemical cell system produces a gas so that the inner pressure increases causing the expansion of the dry cell. Since the tensile strength of the laser-welding system jacket in the radial direction is great, the expansion caused by the increase of the inner pressure takes place in the vertical direction thus producing such troubles as the disengagement of the seal plate which defines the cathode cap provided at the upper part of the dry cell and the spread of the depolarizer mix t the outside. As the result, the appliance incorporated with the dry cell will be heavily damaged, and serious accidents may happen in some cases.
Accordingly, it is a basic object of the present invention to provide an outer metal jacket provided with a safety mechanism so as to prevent a dry cell constructed therewith from its breakage or rupture due to the abnormal rise of its inner pressure. Another object of this invention is to provide a dry cell of high safety using said metal jacket. These and other objects of the invention will be apparent from the foregoing and subsequent descriptions.